The levels of expression of the CLOCK gene were altered in OSAS patients before and after CPAP treatment, suggesting that either the standard therapeutic management of these patients is insufficient to correct the disruption of the circadian patterns of genes with expression conditioned by cycles of sleep/vigilance or changes in severe sleep apnea patients are structural, for example, as a consequence of irreversible cerebral vascular lesions. The gene discussed is CLOCK; the disease is sleep apnea syndrome.